This attempted to pass through double float timestamps in a bit exact
way by reinterpret casting them to int64_t. This usually worked, because
libavcodec (in decoding mode) is mostly not allowed to interpret
timestamps. libavcodec doesn't even know the unit of the timestamps,
unless the API user sets a timebase.
We've stopped doing this, and always set a timebase. Only ad_spdif.c
still used this (indirectly through mp_set_av_packet()), but doesn't
actually need timestamps on the packet. In fact, it's already explicitly
setting the packet timestamp fields to 0 before passing it to FFmpeg
API.
This code is unused, and the passthrough method wasn't terribly elegant
to begin with. Drop this code. Arbitrarily use AV_TIME_BASE_Q as
fallback in situations the passthrough was used.